Communication system, control apparatus, forwarding node, control method and program

ABSTRACT

A plurality of forwarding nodes includes: a topology holding unit that holds as first topology information connection relationship between an own node and at least part of the group of nodes; a topology notification unit that notifies a node(s) adjacent to the own node among the group of nodes of the first topology information; and an in-band communication unit that notifies a control apparatus of the first topology information. The control apparatus includes: a topology configuration unit that generates second topology information by combining a plurality of pieces of the first topology information notified by the plurality of forwarding nodes; and an in-band control unit that derives a path from the control apparatus to the plurality of forwarding nodes based on the second topology information and establishes a control channel for controlling the plurality of forwarding nodes along the derived path.

REFERENCE TO RELATED APPLICATION

The present invention is based upon and claims the benefit of thepriority of Japanese patent application No. 2013-043908 filed on Mar. 6,2013, the disclosure of which is incorporated herein in its entirety byreference thereto.

TECHNICAL FIELD

The present invention relates to a communication system, controlapparatus, forwarding node, control method, and program, andparticularly to a communication system configured so that a controlapparatus centrally manages forwarding nodes provided in a network usingcontrol information and grasps the connection states among theforwarding nodes, a control apparatus and forwarding node in such acommunication system, and a control method and program for controllingthe communication system.

BACKGROUND

As a communication system in which a control apparatus centrallycontrols forwarding nodes, a technology called OpenFlow is known (PatentLiterature 1 and Non-Patent Literatures 1 and 2). OpenFlow treatscommunication as an end-to-end flow and performs path control, failurerecovery, load balancing, and optimization for each flow. An OpenFlowswitch, specified in Non-Patent Literature 2, comprises a secure channelfor communicating with an OpenFlow controller, which can be positionedas a control apparatus, and operates according to a flow table, anaddition to or rewriting of which is suitably instructed by the OpenFlowcontroller. In the flow table, a set of matching rules (Header Field) tobe matched against a packet header, flow statistics (Counters), andactions (Actions) defining the processing contents is defined for eachflow.

For instance, upon receiving a packet, the OpenFlow switch searches foran entry having a matching rule that matches the header information ofthe received packet in the flow table. When an entry matching thereceived packet is found as a result of the search, the OpenFlow switchupdates the flow statistics (Counters) and performs the processingcontents (packet transmission from a designated port, flooding, discard,etc.) written in the action field of the entry on the received packet.On the other hand, when no entry matching the received packet is foundas a result of the search, the OpenFlow switch forwards the receivedpacket to the OpenFlow controller via a secure channel, requests thecontroller to determine a packet path on the basis of the source anddestination of the received packet, receives a flow entry that realizesthis, and updates the flow table. As described, the OpenFlow switchforwards a packet using an entry stored in the flow table as controlinformation.

Non-Patent Literature 3 describes a technology that constructs a securechannel in the OpenFlow network in an actual network using a specialframe and source routing. Hereinafter, a control channel constructed inan actual network is referred to as “in-band secure channel.”

Patent Literature 1:

-   International Publication Number WO/2008/095010    Non-Patent Literature 1:-   Nick McKeown, et al. “OpenFlow: Enabling Innovation in Campus    Networks,” [online], [Searched on Mar. 4, 2013], the Internet <URL:    http://www.openflow.org/documents/openflow-wp-latest.pdf>

Non-Patent Literature 2:

-   “OpenFlow Switch Specification” Version 1.1.0. (Wire Protocol 0x02),    [online], [Searched on Mar. 4, 2013], the Internet    <URL:http://www.openflow.org/documents/openflow-spec-v1.1.0.pdf>

Non-Patent Literature 3:

-   Toshio Koide, Hideyuki Shimonishi, “A study on the automatic    construction mechanism of control network in OpenFlow-based    network,” IEICE Technical Report, The Institute of Electronics,    Information and Communication Engineers, NS2009-165 (2010-3), Vol.    109, No. 448, pp. 19-24, March 2010

SUMMARY Technical Problem

The disclosure of each Patent Literatures and Non-Patent Literaturescited above is incorporated herein in its entirely by reference thereto.The following analysis is given by the present inventor.

By using the method in Non-Patent Literature 3, it is possible toconstruct a network compatible with the OpenFlow at corporations orhomes by using an in-band secure channel without using a networkdedicated to control.

An in-band secure channel is established between a control apparatusthat functions as an OpenFlow controller and a forwarding node thatfunctions as an OpenFlow switch. Non-Patent Literature 3 describes theconstruction of an in-band secure channel using source routing.

The path of an in-band secure channel is a path calculated based on thetopology information of the entire network grasped by the controlapparatus in advance. In an initial state, the control apparatus cannotcommunicate with a forwarding node that is not physically adjacentthereto, and nor can it obtain the control rights over the forwardingnode. Therefore, there is a problem that the control apparatus is unableto discover the topology information of the entire network.

In order to resolve this problem, Non-Patent Literature 3 describes amethod in which the topology information of the entire network isdiscovered by obtaining the control rights over a forwarding nodeadjacent to the control apparatus, the repeating the operation ofobtaining the control rights over a forwarding node adjacent to theforwarding node, and ultimately obtaining the control rights over allforwarding nodes.

However, according to the method in Non-Patent Literature 3, it takes along time to obtain the control rights over all forwarding nodes sincethe topology is discovered sequentially, and there is a risk that theamount of the control information that travels over the network will beenormous. Further, according to this method, when there is a change inthe network topology due to the addition or deletion of a forwardingnode, or the addition, deletion or failure of a communication linkbetween the forwarding nodes, it is difficult to respond to the changeimmediately. Further, when the change is responded immediately, a loadis applied on the network due to a large amount of control information,and the network performance may deteriorate.

Therefore, it is desired that the control apparatus reduces the timerequired to discover the topology of the entire network and reduces thetransmission/reception loads of the control information required todiscover the topology.

According to a first aspect of the present invention, there is provideda communication system, comprising: a group of nodes including aplurality of forwarding nodes and a control apparatus that controls theplurality of forwarding nodes. The plurality of forwarding nodescomprises: a topology holding unit that holds as first topologyinformation connection relationship between an own node and at leastpart of the group of nodes; a topology notification unit that notifies anode(s) adjacent to the own node among the group of nodes of the firsttopology information; and an in-band communication unit that notifiesthe control apparatus of the first topology information. The controlapparatus comprises: a topology configuration unit that generates secondtopology information by combining a plurality of pieces of the firsttopology information notified by the plurality of forwarding nodes; andan in-band control unit that derives a path from the control apparatusto the plurality of forwarding nodes based on the second topologyinformation and establishes a control channel for controlling theplurality of forwarding nodes along the derived path.

According to a second aspect of the present invention, there is provideda forwarding node among a plurality of forwarding nodes controlled by acontrol apparatus. The forwarding node comprises: a topology holdingunit that holds as first topology information connection relationshipbetween an own node and at least part of a group of nodes including thecontrol apparatus and the plurality of forwarding nodes; a topologynotification unit that notifies a node(s) adjacent to the own node amongthe group of nodes of the first topology information; and an in-bandcommunication unit that notifies the control apparatus of the firsttopology information.

According to a third aspect of the present invention, there is provideda control apparatus for controlling a plurality of forwarding nodes. Theplurality of forwarding nodes hold as first topology informationconnection relationship between an own node and at least part of a groupof nodes including the plurality of forwarding nodes and the controlapparatus, and notify the control apparatus and a node(s) adjacent tothe own node among the group of nodes of the first topology information.The control apparatus comprises: a topology configuration unit thatgenerates second topology information by combining a plurality of piecesof the first topology information notified by the plurality offorwarding nodes; and an in-band control unit that derives a path fromthe control apparatus to the plurality of forwarding nodes based on thesecond topology information and establishes a control channel forcontrolling the plurality of forwarding nodes along the derived path.

According to a fourth aspect of the present invention, there is provideda control method for controlling a plurality of forwarding nodes using acontrol apparatus. The control method comprises: by the plurality offorwarding nodes, holding as first topology information connectionrelationship between an own node and at least part of a group of nodesincluding the plurality of forwarding nodes and the control apparatus;notifying a node(s) adjacent to the own node among the group of nodes ofthe first topology information; notifying the control apparatus of thefirst topology information. Further, the control method comprises: bythe control apparatus, generating second topology information bycombining a plurality of pieces of the first topology informationnotified by the plurality of forwarding nodes; deriving a path from thecontrol apparatus to the plurality of forwarding nodes based on thesecond topology information; and establishing a control channel forcontrolling the plurality of forwarding nodes along the derived path.

According to a fifth aspect of the present invention, there is provideda control method, comprising: by one of a plurality of forwarding nodescontrolled by a control apparatus, holding as first topology informationconnection relationship between an own node and at least part of a groupof nodes including the control apparatus and the plurality of forwardingnodes; notifying a node(s) adjacent to the own node among the group ofnodes of the first topology information; and notifying the controlapparatus of the first topology information.

The present invention provides the following advantages, but notrestricted thereto. According to the communication system, forwardingnode, control apparatus and control method relating to the presentinvention, it becomes possible to shorten the time required for acontrol apparatus to discover the topology of the entire network and toreduce the transmission/reception loads of control information requiredto discover the topology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a communicationsystem relating to an exemplary embodiment as an example.

FIG. 2 is a block diagram showing a configuration of a communicationsystem of a first exemplary embodiment as an example.

FIG. 3 is a drawing for explaining an example of an operation of thecommunication system relating to the first exemplary embodiment.

FIG. 4 is a drawing showing a configuration of transmission topologyinformation and reception topology information as an example.

FIG. 5 is a drawing showing a list of topology information grasped by acontrol apparatus and a forwarding node in an initial state.

FIG. 6 is a drawing showing transmission topology information 30Atransmitted from each of forwarding nodes 20A to 20C in an initialstate.

FIG. 7 is a drawing showing reception topology information 30B receivedby a control apparatus 10A and the forwarding nodes 20A to 20C.

FIG. 8 is a drawing showing topology information configured and held bya topology configuration unit.

FIG. 9 is a drawing showing topology information grasped by the controlapparatus 10A.

FIG. 10 is a drawing showing topology information grasped by the controlapparatus 10A.

FIG. 11 is a block diagram showing a configuration of a communicationsystem relating to a second exemplary embodiment as an example.

FIG. 12 is a drawing for explaining an example of an operation of thecommunication system relating to a second exemplary embodiment.

PREFERRED MODES

In the present disclosure, there are various possible modes, whichinclude the following, but not restricted thereto. First, a summary ofan exemplary embodiment will be described. Note that drawing referencesigns in the summary are given solely to facilitate understanding forconvenience and are not intended to limit the present invention to themodes shown in the drawings.

FIG. 1 shows an example of the configuration of a communication systemrelating to an exemplary embodiment. In FIG. 1, the communication systemcomprises a group of nodes including a plurality of forwarding nodes(20A to 20D) and a control apparatus (10A) that controls the pluralityof forwarding nodes (20A to 20D). The plurality of forwarding nodes (20Ato 20D) comprise a topology holding unit (21) that holds as firsttopology information connection relationship between the own node and atleast part the group of nodes, a topology notification unit (25) thatnotifies the first topology information to a node(s) among the group ofnodes adjacent to the own node, and an in-band communication unit (22)that notifies the first topology information to the control apparatus(10A).

The control apparatus (10A) comprises a topology configuration unit (11)that integrates a plurality of pieces of the first topology informationnotified by the plurality of forwarding nodes (20A to 20D) to generatesecond topology information, and an in-band control unit (12) thatderives a path from the control apparatus (10A) to the plurality offorwarding nodes based on the second topology information andestablishes a control channel for controlling the plurality offorwarding nodes along the derived path.

The topology holding unit (for instance, the topology holding unit 21 ofthe forwarding node 20A) may update the first topology information heldin the own node based on the first topology information received fromthe node (20B) adjacent to the local node (20A) among the plurality offorwarding nodes (20A to 20D). Further, the topology holding unit (forinstance, the topology holding unit 21 of the forwarding node 20A) mayhold as the first topology information connection relationship betweenthe own node (20A) and nodes (10A and 20B) within a predetermined hopnumber (for instance 1 hop) from the local node (20A) among the group ofnodes. Further, the topology holding unit (for instance the topologyholding unit 21 of the forwarding node 20A) may hold as the firsttopology information connection relationship between the own node (20A)and a node(s) within a predetermined physical distance from the localnode (20A) among the group of nodes or a node(s) of which thecommunication delay from the own node (20A) is within a predeterminedperiod of time.

The topology notification unit (for instance, the topology notificationunit 25 of the forwarding node 20) may use any of the following as acondition (a transmission timing condition) for notifying the firsttopology information to nodes (10A and 20B) adjacent to the own node(20A) among the group of nodes: the first topology information held bythe topology holding unit (21) has been updated, a predetermined timeperiod has passed since an update, a predetermined time period haspassed since the previous notification, and any combination of theabove.

According to such a communication system, it is possible to reduce thetime required for the control apparatus (10A) to discover the topologyof the entire network and reduce the transmission/reception loads ofcontrol information required to discover the topology, compared with therelated technology (Non-Patent Literature 3) described in Background.

In addition, the control apparatus (10A) may comprise a conditioninstruction unit (not shown in FIG. 1) that instructs a predeterminednumber of hops (or predetermined distance or time period) to theplurality of forwarding nodes (20A to 20D) via an established controlchannel. Further, the control apparatus (10A) may comprise a conditioninstruction unit that instructs the above conditions (transmissiontiming conditions) to the plurality of forwarding nodes (20A to 20D) viathe established control channel.

According to such a communication system, it is possible for the controlapparatus (10A) to centrally manage the control of the amount of packetsexchanged to discover the state of the topology and the control of thetime required to discover any change in the topology, facilitating thereflection of the network operation policy of a network operator.

The communication system relating to the present invention will bedescribed in detail using exemplary embodiments below.

Exemplary Embodiment 1

A communication system relating to a first exemplary embodiment will bedescribed in detail with reference to the drawings. FIG. 2 is a blockdiagram showing the configuration of the communication system relatingto the present exemplary embodiment as an example. FIG. 2 shows that thecommunication system comprises forwarding nodes 20A to 20D and a controlapparatus 10A that controls the forwarding nodes 20A to 20D.

FIG. 2 illustrates a case where the communication system comprises fourforwarding nodes 20A to 20D. FIG. 2 also illustrates a case where thecontrol apparatus 10A is connected to two forwarding nodes 20A and 20C.Note that the connection relationship of the forwarding nodes and thenumber of forwarding nodes are not limited to the mode illustrated inFIG. 2. The control apparatus 10A needs to have at least one forwardingnode connected thereto, and the number of forwarding nodes connected tothe control apparatus 10A may be one, three or greater.

The control apparatus 10A comprises a topology configuration unit 11that generates topology information by discovering connectioninformation among the forwarding nodes 20A to 20D, an in-band controlunit 12 that establishes a channel to the forwarding nodes 20A to 20Dand transmits/receives control information, and a packet processing unit13 that transmits, receives and distributes a packet.

The topology configuration unit 11 requests topology information graspedby the forwarding nodes 20A to 20D via the in-band control unit 12 andreceives a topology information response as a reply thereto. Further,the topology configuration unit 11 discovers and holds the topologyinformation of the entire network by assembling partial pieces oftopology information included in the topology information responsesreceived. Further, the topology configuration unit 11 updates and holdsthe topology information of the entire network on the basis of topologyinformation supplied by the packet processing unit 13. Also, thetopology configuration unit 11 replies the topology information heldtherein in response to a reference request from the in-band control unit12.

The in-band control unit 12 grasps the presence of the forwarding nodes20A to 20D by referring to the topology information of the entirenetwork held by the topology configuration unit 11. Moreover, thein-band control unit 12 calculates a forwarding path for the channelestablished between each forwarding node and the control apparatus 10A,and establishes a control channel along each forwarding path.

The packet processing unit 13 transmits/receives a packet to/from theforwarding nodes 20A and 20C connected to the control apparatus 10A anddistributes only a packet required for the in-band control unit 12 fromthe transmitted/received packets to the in-band control unit 12. Inaddition, when receiving a piece of topology information, the packetprocessing unit 13 configures the topology information with informationof a communication port that received it, and outputs the result to thetopology configuration unit 11.

Each of the forwarding nodes 20A to 20D comprises a topology holdingunit 21 that holds surrounding topology information known to each node,an in-band communication unit 22 that establishes a channel with thecontrol apparatus 10A and transmits/receives control information, apacket processing unit 23 that transmits, receives and distributes apacket, a topology filter unit 24 that reduces the amount of topologyinformation to a predetermined range, and a topology notification unit25 that notifies topology information to surrounding forwarding nodes.

The topology holding unit 21 holds surrounding topology information ofthe local node, provides the held topology information in response to arequest from the in-band communication unit 22 and the topologynotification unit 25, and combines and updates topology informationsupplied by the topology filter unit 24 and the held topologyinformation. In addition, when there is a change in the topology, thetopology holding unit 21 notifies the topology notification unit 25 ofthe change. In an initial state, the topology holding unit 21 holdstopology information constituted by only information of the local node.

The in-band communication unit 22 establishes a control channel with thecontrol apparatus 10A according to control of the control apparatus 10Aand instructs the packet processing unit 23 to forward a control channelof another forwarding node in accordance with control of the controlapparatus 10A. Further, the in-band communication unit 22 makes aninquiry about topology information to the topology holding unit 21 viathe packet processing unit 23 in response to a topology informationrequest received from the control apparatus 10A. Moreover, the in-bandcommunication unit 22 transmits the obtained topology information to thecontrol apparatus 10A via the packet processing unit 23 as a topologyinformation response.

The packet processing unit 23 transmits/receives a packet from/to theother forwarding nodes or the control apparatus 10A. Further, whenreceiving topology information, the packet processing unit 23 outputs itto the topology filter unit 24 with the information of a communicationport that received the topology information. Further, the packetprocessing unit 23 supplies/outputs a packet transmitted/receivedto/from the control apparatus 10A to the in-band communication unit 22.In addition, the packet processing unit 23 transmits topologyinformation supplied by the topology notification unit 25 withforwarding node information and the communication port information usedfor transmission. Further, the packet processing unit 23 forwards acontrol channel of another forwarding node in accordance with aninstruction from the in-band communication unit 22.

The topology filter unit 24 reduces the amount of the topologyinformation supplied by the packet processing unit 23 to a predeterminedrange and outputs the result to the topology holding unit 21. Thetopology filter unit 24 may use a filter condition, for instance settingthe predetermined range as nodes within a predetermined number of hopssurrounding the local node. In this case, the topology filter unit 24reduces the supplied topology information to the topology constituted byonly the topology of nodes within the predetermined number of hopssurrounding the local node. Note that the filter condition created andapplied by the topology filter unit 24 is not limited thereto. It ispossible to determine the attributes of the topology information held bythe topology holding unit 21 according to the filter condition.

The topology notification unit 25 obtains the topology information heldby the topology holding unit 21 and instructs the packet processing unit23 to notifies surrounding forwarding nodes of the information. Further,if a control channel with the control apparatus 10A has beenestablished, the topology notification unit 25 can notify the controlapparatus 10A of the topology information held by the topology holdingunit 21. As for the timing when the topology notification unit 25outputs the topology information (a transmission timing condition), thetopology notification unit 25 may output it immediately after thetopology holding unit 21 is updated or when there is no update for apredetermined period of time after the last update. Further, regardlessof the presence of an update, the topology notification unit 25 maytransmit topology information on a regular basis. Any transmissiontiming condition can be created and applied, and it is possible tocontrol the total number of packets discovering topology information andthe convergence of topology information discovery according to thetransmission timing condition.

As described above, the communication system relating to the presentexemplary embodiment comprises the plurality of forwarding nodes 20A to20D that partially discover the topology and transmits/receives controlinformation and the control apparatus 10A that controls the forwardingnodes using the control information. The control apparatus 10A comprisesthe topology configuration unit 11 that discovers and configures theconnection information among the forwarding nodes, and the in-bandcontrol unit 12 that determines a forwarding path for controlinformation for each forwarding node on the basis of the information ofthe topology configuration unit 11 and establishes a control channel inaccordance with each forwarding path. In addition, each forwarding nodecomprises the topology holding unit 21 that holds surrounding topologyinformation known to each node, the topology notification unit 25 thatnotifies surrounding forwarding nodes of this topology information, thetopology filter unit 24 that reduces the amount of the received topologyinformation to a predetermined range and registers the result in thetopology holding unit 21, and the in-band communication unit 22 thatreplies the topology information to the control apparatus 10A inresponse to a request from the control apparatus 10A.

According to the communication system, it is possible to shorten thetime required to discover the topology of the entire network needed in acase where a centralized control network represented by OpenFlow isconstructed and to reduce the transmission/reception loads of controlinformation required to discover the topology.

Next, the operation of the communication system (FIG. 1) of the presentexemplary embodiment will be described using a concrete example in whichthree forwarding nodes 20A to 20C and the control apparatus 10A areconnected in series.

FIG. 3 is a drawing for explaining the operation of the communicationsystem relating to the present exemplary embodiment. FIG. 3 shows thatthe control apparatus 10A and the three forwarding nodes 20A to 20C areconnected in series in the communication system. In FIG. 3, signs #1 and#2 given between the control apparatus 10A and the three forwardingnodes 20A to 20C indicate the identifiers of communication ports.

Here, the contents of a packet transmitted by the forwarding nodes 20Ato 20C will be described. The packet processing unit 23 of eachforwarding node receives and outputs a packet including the informationof a communication port used and the information of a source forwardingnode when topology information is transmitted/received. FIG. 4 showsinformation included in transmission topology information 30A includedin a transmitted packet and information included in reception topologyinformation 30B constituted by a received packet. Further, how pieces ofinformation in the transmission topology information 30A and in thereception topology information 30B are arranged is not limited to themode shown in FIG. 4. Further, the transmission topology information 30Aand the reception topology information 30B may include information otherthan the information shown in FIG. 4.

The transmission topology information 30A is constituted when topologyinformation is transmitted. The transmission topology information 30Aincludes pieces of information, each representing topology information31A, a source forwarding node 32A, and a source communication port 33A.The reception topology information 30B is constituted when topologyinformation is received. The reception topology information 30B includesa piece of information representing a destination communication port 34Bin addition to the information included in the transmission topologyinformation 30A.

FIG. 5 is a drawing showing a list of topology information grasped bythe control apparatus 10A and the forwarding nodes 20A to 20C in aninitial state. FIG. 5 shows that, in an initial state, each topologyholding unit 21 of the forwarding nodes 20A to 20C and the topologyconfiguration unit 11 of the control apparatus 10A hold topologyinformation that does not include the other forwarding nodes or thecontrol apparatus 10A although each topology information includes thelocal node.

Each topology notification unit 25 of the forwarding nodes 20A to 20Cstarts a periodic operation, asks the topology holding unit 21 fortopology information, obtains the information, and configures a packetthat includes the transmission topology information 30A including thetopology information acquired via the packet processing unit 23. Thetopology notification unit 25 transmits a configured packet to a nodeadjacent to the local node, out of the control apparatus 10A and theforwarding nodes 20A to 20C.

FIG. 6 shows the transmission topology information 30A transmitted fromeach of the forwarding node 20A, 20B, and 20C in an initial state. FIG.6 shows that the forwarding node 20A configures the transmissiontopology information 30A that includes topology information includingonly the forwarding node 20A for the communication port #2 and transmitsa packet 40 that includes a communication port #A from the communicationport #2. In addition, the forwarding node 20A configures thetransmission topology information 30A that includes topology informationincluding only the forwarding node 20A for the communication port #1 andtransmits a packet 41 that includes the communication port #1 from thecommunication port #1. Similarly, the forwarding node 20B transmitspackets 42 and 43 from the communication port #2 and #1, respectively.Further, the forwarding node 20C also transmits a packet 44 from thecommunication port #2.

The control apparatus 10A receives the packet 40 transmitted from theforwarding node 20A. The packet processing unit 13 configures thereceived packet 40 as the reception topology information 30B.

FIG. 7 shows the reception topology information 30B received by thecontrol apparatus 10A and the forwarding nodes 20A to 20C. Further, FIG.8 shows a list of topology information grasped by the control apparatus10A and the forwarding nodes 20A to 20C after the packets 40 to 44 arereceived.

The first line in FIG. 7 shows the reception topology information 30Bconstituted by the packet 40 received by the control apparatus 10A. Thefirst line in FIG. 7 shows that a source forwarding node 32B is theforwarding node 20A, a source communication port 33B is thecommunication port #2, and the destination communication port 34B is thecommunication port #1. At this time, the packet processing unit 13discovers that the communication port #2 of the forwarding node 20A andthe communication port #1 of the control apparatus 10A are connected bya communication link and outputs this fact as a piece of topologyinformation to the topology configuration unit 11.

The topology configuration unit 11 receives the topology informationfrom the packet processing unit 13, and then configures and holdstopology information in which the communication port #2 of theforwarding node 20A and the communication port #1 of the controlapparatus 10A are connected by a communication link, as shown in FIG. 8.

Similarly, the forwarding node 20A receives the packet 42 transmitted bythe forwarding node 20B. The packet processing unit 23 configures thereceived packet 42 as the reception topology information 30B. The thirdline of FIG. 7 shows the reception topology information 30B configuredfrom the received packet 42. The third line of FIG. 7 shows that thesource forwarding node is the forwarding node 20B, the sourcecommunication port is the communication port #2, and the destinationcommunication port is the communication port #1. At this time, thepacket processing unit 23 of the forwarding node 20A discovers that thecommunication port #2 of the forwarding node 20B and the communicationport #1 of the forwarding node 20A are connected by a communicationlink, and outputs this fact as a piece of topology information to thetopology filter unit 24.

The topology filter unit 24 of the forwarding node 20A processes thetopology information supplied by the packet processing unit 23 on thebasis of a specified filter condition. Here, a filter condition isassumed to dictate that the topology up to the forwarding nodes adjacentto the local node should be held. In this case, the supplied topologyinformation fits into such a filter condition. Therefore, the topologyfilter unit 24 outputs the topology information to the topology holdingunit 21 without processing the topology information.

When receiving the topology information from the topology filter unit24, the topology holding unit 21 of the forwarding node 20A configuresand holds topology information in which the communication port #2 of theforwarding node 20B and the communication port #1 of the forwarding node20A are connected by a communication link, as shown in FIG. 8.

The forwarding node 20B receives the packet 41 transmitted by theforwarding node 20A and the packet 44 transmitted by the forwarding node20C. The forwarding node 20C receives the packet 43 transmitted by theforwarding node 20B. Each of the forwarding nodes 20B and 20C alsooperates similarly as the forwarding node 20A, and configures and holdstopology information as shown in FIG. 8.

The topology configuration unit 11 of the control apparatus 10Adiscovers the presence of the forwarding node 20A by updating thetopology information and instructs the in-band control unit 12 toestablish a control channel. As a method for establishing a controlchannel between the control apparatus 10A and the forwarding node 20A,the method described in Non-Patent Literature 3 can be used forinstance. However, the method for establishing a control channel is notlimited thereto.

The topology configuration unit 11 requests topology information fromthe forwarding node 20A using the control channel established betweenthe forwarding node 20A and the control apparatus 10A. This topologyinformation request is supplied to the in-band communication unit 22 ofthe forwarding node 20A via the in-band control unit 12 and theestablished control channel.

The in-band communication unit 22 of the forwarding node 20A makes aninquiry on topology information to the topology holding unit 21according to the supplied topology information request and returns atopology information response including the acquired topologyinformation. The topology information response is supplied to thein-band control unit 12 of the control apparatus 10A via the establishedcontrol channel with the control apparatus 10A.

The in-band control unit 12 of the control apparatus 10A outputstopology information included in the supplied topology informationresponse to the topology configuration unit 11. The topologyconfiguration unit 11 combines the supplied topology information andtopology information currently held by the topology configuration unit11 and discovers a wider range of topology information.

FIG. 9 is a drawing showing the topology information grasped by thecontrol apparatus 10A. FIG. 9 shows that the control apparatus 10A isable to grasp topology information encompassing the control apparatus10A and the forwarding nodes 20A and 20B at this point.

Similarly, the topology configuration unit 11 of the control apparatus10A discovers the presence of the forwarding node 20B by updating thetopology information. Next, the topology configuration unit 11 instructsthe in-band control unit 12 to establish a control channel of theforwarding node 20B and transmits a topology information request. As aresult, the topology configuration unit 11 receives the topologyinformation of the forwarding node 20B as a topology informationresponse, combines the received information with the topologyinformation currently held by the topology configuration unit 11, anddiscovers the topology information of the entire network.

FIG. 10 is a drawing showing the topology information grasped by thecontrol apparatus 10A. FIG. 10 shows that the control apparatus 10A isable to grasp the topology information of the entire network at thispoint.

Similarly, the topology configuration unit 11 of the control apparatus10A discovers the presence of the forwarding node 20C by updating thetopology information. Next, the topology configuration unit 11 instructsthe in-band control unit 12 to establish a control channel of theforwarding node 20C.

As described, the control apparatus 10A can establish the controlchannels with all the forwarding nodes 20A to 20C.

Thereafter, unless a topology change occurs, the forwarding nodes 20Aand 20C maintain the state, in which the topology up to adjacent nodesare held, using a condition, set by the topology filter unit 24 as afilter condition, that the topology up to the forwarding nodes adjacentto the local node should be held.

According to the present exemplary embodiment, each forwarding node isable to maintain the latest state of the topology information up to thenodes adjacent to the local node. As a result, if there is any topologychange in a certain area of the network, it is possible to notify thechange, reduce the amount of packet exchanges for discovering the latesttopology state, thereby reducing the load of the entire network.Further, the control apparatus 10A is able to discover any change in thetopology via the control channel, and the notifications thereof arelimited to those from the forwarding nodes surrounding the area wherethe topology change has occurred. Therefore, the control apparatus 10Ais able to discover any change in the topology immediately and is ableto further reduce the load of the entire network by reducing the amountof packet exchanges.

Exemplary Embodiment 2

Next, a communication system relating to a second exemplary embodimentwill be described in detail with reference to the drawings. FIG. 11 is ablock diagram showing the configuration of the communication systemrelating to the present exemplary embodiment as an example. FIG. 11shows that the communication system relating to the present exemplaryembodiment comprises forwarding nodes 60A to 60D and a control apparatus50A that controls the forwarding nodes 60A to 60D as in thecommunication system relating to the first exemplary embodiment.

The control apparatus 50A further comprises a condition instruction unit54, in addition to the configuration of the control apparatus 10A of thefirst exemplary embodiment (FIG. 2).

A topology configuration unit 51 operates in the same manner as thetopology configuration unit 11 of the first exemplary embodiment, exceptthat the topology configuration unit 51 replies topology informationheld therein in response to a reference request from the conditioninstruction unit 54.

An in-band control unit 52 and a packet processing unit 53 operate inthe same manner as the in-band control unit 12 and the packet processingunit 13, respectively, in the control apparatus 10A of first exemplaryembodiment.

The condition instruction unit 54 discovers the presence of theforwarding nodes 60A to 60D by referring to the topology information ofthe entire network held by the topology configuration unit 51. Further,the condition instruction unit 54 specifies for each of the forwardingnode 60A to 60D any one or both of a filter condition to be set in atopology filter unit 64 of the forwarding nodes 60A to 60D and atransmission timing condition to be set in a topology notification unit65 via a control channel established by the in-band control unit 52.

The forwarding nodes 60A to 60D are configured identically to theforwarding nodes 20A to 20D relating to the first exemplary embodiment.

An in-band communication unit 62 operates in the same manner as thein-band communication unit 22 in the first exemplary embodiment, exceptthat the in-band communication unit 62 outputs a filter conditionspecified by the condition instruction unit 54 of the control apparatus50A to the topology filter unit 64 and outputs a transmission timingcondition to the topology notification unit 65.

The topology filter unit 64 operates in the same manner as the topologyfilter unit 24 in the first exemplary embodiment, except that thetopology filter unit 64 applies the filter condition supplied by thein-band communication unit 62.

The topology notification unit 65 operates in the same manner as thetopology notification unit 25 in the first exemplary embodiment, exceptthat the topology notification unit 65 applies the transmission timingcondition supplied by the in-band communication unit 62.

A topology holding unit 61 and a packet processing unit 63 operate inthe same manner as the topology holding unit 21 and the packetprocessing unit 23, respectively, in the first exemplary embodiment.

Next, the operation of the communication system (FIG. 11) of the presentexemplary embodiment will be described using a concrete example in whichthree forwarding nodes 60A to 60C and the control apparatus 50A areconnected in series.

FIG. 12 is a drawing for explaining the operation of the communicationsystem relating to the present exemplary embodiment. FIG. 12 shows thatthe communication system has the control apparatus 50A and the threeforwarding nodes 60A to 60C connected in series. In FIG. 12, signs #1and #2 given among the control apparatus 50A and the three forwardingnodes 60A to 60C indicate the identifiers of communication ports.

In the similar manner as the operation of the communication systemrelating to the first exemplary embodiment described with references toFIGS. 3 to 10, the control apparatus 50A is able to ultimately establishcontrol channels with all of the forwarding nodes 60A to 60C.

The condition instruction unit 54 of the control apparatus 50A transmitsfilter conditions and transmission timing conditions appropriately setfor each of the forwarding nodes 60A to 60C to each of the forwardingnodes 60A to 60C via the control channels established by the in-bandcontrol unit 52.

The in-band communication unit 62 of the forwarding node 60A outputs thefilter condition received via the control channel to the topology filterunit 64. Further, the in-band communication unit 62 outputs thetransmission timing condition received via the control channel to thetopology notification unit 65. The topology filter unit 64 of theforwarding node 60A holds the supplied filter condition and applies thecondition for filtering the subsequent topology information. Thetopology notification unit 65 of the forwarding node 60A holds thesupplied transmission timing condition and applies it as a condition forcontrolling the timing of subsequent topology notifications to thesurrounding forwarding nodes and the control apparatus 50A.

Similarly, each of the forwarding nodes 60B and 60C also holds andapplies specified filter conditions and transmission conditions.

According to the communication system of the present exemplaryembodiment, the control apparatus 50A is able to control the filtercondition and the transmission timing condition for each forwarding nodethat exists in the network. As a result, the control apparatus 50A isable to centrally manage the control of the amount of packet exchangesfor discovering the state of the topology and the control of the timerequired to discover any topology change. Consequently, it facilitatesthe reflection of the network operation policy of a network operator.

Further, modes described below as modes are possible in the presentinvention.

(Mode 1)

-   A communication system may be the communication system according to    the first aspect of the present invention.    (Mode 2)-   The communication system according to mode 1, wherein the topology    holding unit updates the first topology information held in the own    node based on the first topology information received from the    node(s) adjacent to the own node among the plurality of forwarding    nodes.    (Mode 3)-   The communication system according to mode 2, wherein the topology    holding unit holds as the first topology information connection    relationship between the own node and a node(s) within a    predetermined hop number from the own node among the group of nodes.    (Mode 4)-   The communication system according to mode 2, wherein the topology    notification unit uses, as a condition for notifying the first    topology information to the node(s) adjacent to the own node among    the group of nodes, at least one of:-   the first topology information held by the topology holding unit has    been updated;-   a predetermined time period has passed since the update; and-   a predetermined time period has passed since a previous    notification.    (Mode 5)-   The communication system according to mode 3, wherein-   the control apparatus further comprises a condition instruction unit    that specifies the predetermined hop number for the plurality of    forwarding nodes via the control channel.    (Mode 6)-   The communication system according to mode 4, wherein the control    apparatus further comprises a condition instruction unit that    specifies the condition for the plurality of forwarding nodes via    the control channel.    (Mode 7)-   A forwarding node may be the forwarding node according to the second    aspect of the present invention.    (Mode 8)-   The forwarding node according to mode 7, wherein-   the topology holding unit updates the first topology information    held in the own node based on the first topology information    received from the node(s) adjacent to the own node among the    plurality of forwarding nodes.    (Mode 9)-   The forwarding node according to mode 8, wherein-   the topology holding unit holds as the first topology information    connection relationship between the own node and a node(s) within a    predetermined hop number from the own node among the group of nodes.    (Mode 10)-   The forwarding node according to mode 8, wherein the topology    notification unit uses, as a condition for notifying the first    topology information to the node(s) adjacent to the own node among    the group of nodes, at least one of:-   the first topology information held by the topology holding unit has    been updated;-   a predetermined time period has passed since the update; and-   a predetermined time period has passed since a previous    notification.    (Mode 11)-   The forwarding node according to mode 9, configured to receive an    instruction of the predetermined hop number from the control    apparatus via the control channel.    (Mode 12)-   The forwarding node according to mode 10, configured to receive an    instruction of the condition from the control apparatus via the    control channel.    (Mode 13)-   A control apparatus may be the control apparatus according to the    third aspect of the present invention.    (Mode 14)-   The control apparatus according to mode 13, wherein-   the plurality of forwarding nodes update the first topology    information held in the own node based on the first topology    information received from the node(s) adjacent to the own node among    the plurality of forwarding nodes.    (Mode 15)-   The control apparatus according to mode 13, wherein-   the plurality of forwarding nodes hold as the first topology    information connection relationship between the own node and a    node(s) within a predetermined hop number from the own node among    the group of nodes.    (Mode 16)-   The control apparatus according to mode 14, wherein-   the plurality of forwarding nodes use. as a condition for notifying    the first topology information to the node(s) adjacent to the own    node among the group of nodes, at least one of:-   the first topology information held by the topology holding unit has    been updated;-   a predetermined time period has passed since the update; and-   a predetermined time period has passed since a previous    notification.    (Mode 17)-   The control apparatus according to mode 15, further comprising:-   a condition instruction unit that specifies the predetermined hop    number for the plurality of forwarding nodes via the control    channel.    (Mode 18)-   The control apparatus according to mode 16, further comprising:-   a condition instruction unit that specifies the condition for the    plurality of forwarding nodes via the control channel.    (Mode 19)-   A control method may be the control method according to the fourth    aspect of the present invention.    (Mode 20)-   The control method according to mode 19, comprising:-   by the plurality of forwarding nodes, updating the first topology    information held in the own node based on the first topology    information received from the node(s) adjacent to the own node among    the plurality of forwarding nodes.    (Mode 21)-   The control method according to mode 20, wherein-   the plurality of forwarding nodes hold as the first topology    information connection relationship between the own node and a    node(s) within a predetermined hop number from the own node among    the group of nodes.    (Mode 22)-   The control method according to mode 20, wherein-   the plurality of forwarding nodes use as a condition for notifying    the first topology information to the node(s) adjacent to the own    node among the group of nodes, at least one of:-   the first topology information held by the topology holding unit has    been updated;-   a predetermined time period has passed since the update; and-   a predetermined time period has passed since a previous    notification.    (Mode 23)-   A control method may be the control method according to the fifth    aspect of the present invention.    (Mode 24)-   A control method, comprising:-   by a control apparatus that controls a plurality of forwarding    nodes, generating second topology information by combining a    plurality of pieces of first topology information notified by the    plurality of forwarding nodes; and-   deriving a path from the control apparatus to the plurality of    forwarding nodes based on the second topology information and    establishing a control channel for controlling the plurality of    forwarding nodes along the derived path, wherein-   the plurality of forwarding nodes hold as the first topology    information connection relationship between an own node and at least    part of a group of nodes including the plurality of forwarding nodes    and the control apparatus, and notify the control apparatus and a    node(s) adjacent to the own node among the group of nodes of the    first topology information.    (Mode 25)-   A program, causing a computer provided in one of a plurality of    forwarding nodes controlled by a control apparatus to execute:-   holding as first topology information connection relationship    between an own node and at least part of a group of nodes including    the control apparatus and the plurality of forwarding nodes;-   notifying a node(s) adjacent to the own node among the group of    nodes of the first topology information; and-   notifying the control apparatus of the first topology information.    (Mode 26)-   A program, causing a computer provided in a control apparatus that    controls a plurality of forwarding nodes to execute:-   generating second topology information by combining a plurality of    pieces of first topology information notified by the plurality of    forwarding nodes; and-   deriving a path from the control apparatus to the plurality of    forwarding nodes based on the second topology information, and    establishing a control channel for controlling the plurality of    forwarding nodes along the derived path, wherein-   the plurality of forwarding nodes hold as the first topology    information connection relationship between an own node and at least    part of a group of nodes including the plurality of forwarding nodes    and the control apparatus, and notify the control apparatus and a    node(s) adjacent to the own node among the group of nodes of the    first topology information.    (Mode 27)-   A non-transitory computer-readable storage medium holding the    program according to mode 25 or 26.

Further, the disclosure of each Patent Literature and Non-PatentLiterature cited above is incorporated herein in its entirety byreference thereto. It should be noted that other objects, features andaspects of the present invention will become apparent in the entiredisclosure and that modifications may be done without departing the gistand scope of the present invention as disclosed herein and claimed asappended herewith. Also it should be noted that any combination of thedisclosed and/or claimed elements, matters and/or items may fall underthe modifications. Particularly, the ranges of the numerical values usedin the present description should be interpreted as a specific numericvalue or small range included in the ranges even in cases where it isnot stated so.

-   10A, 50A: control apparatus-   11, 51: topology configuration unit-   12, 52: in-band control unit-   13, 53: packet processing unit-   20A to 20D, 60A to 60D: forwarding nodes-   21, 61: topology holding unit-   22, 62: in-band communication unit-   23, 63: packet processing unit-   24, 64: topology filter unit-   25, 65: topology notification unit-   30A: transmission topology information-   30B: reception topology information-   31A, 31B: topology information-   32A, 32B: transmission source forwarding nodes-   33A, 33B: transmission source communication port-   34B: destination communication port-   40 to 44: packet-   54: condition instruction unit

What is claimed is:
 1. A communication system, comprising: a group ofnodes including a plurality of forwarding nodes and a control apparatusthat controls the plurality of forwarding nodes, wherein the pluralityof forwarding node comprises: a topology holding unit that holds asfirst topology information connection relationship between an own nodeand at least a part of the group of nodes; a topology notification unitthat notifies a node(s) adjacent to the own node among the group ofnodes of the first topology information; and an in-band communicationunit that notifies the control apparatus of the first topologyinformation, and wherein the control apparatus comprises; a topologyconfiguration unit that generates second topology information bycombining a plurality of pieces of the first topology informationnotified by the plurality of forwarding nodes; and an in-band controlunit derives a path from the control apparatus to the plurality offorwarding nodes based on the second topology information andestablishes a control channel for controlling the plurality offorwarding nodes along the derived path, wherein the topology holdingunit updates the first topology information held in the own node basedon the first topology information received from the node(s) adjacent tothe own node among the plurality of forwarding nodes, and wherein thetopology holding unit holds, as the first topology information, aconnection relationship between the own node and a node(s) within apredetermined hop number from the own node among the group of nodes. 2.The communication system according to claim 1, wherein the controlapparatus further comprises a condition instruction unit that specifiesthe predetermined hop number for the plurality of forwarding nodes viathe control channel.
 3. A communication system, comprising: a group ofnodes including a plurality of forwarding nodes and a control apparatusthat controls the plurality of forwarding nodes, wherein the pluralityof forwarding nodes comprises: a topology holding unit that holds asfirst topology information connection relationship between an own nodeand at least a part of the group of nodes; a topology notification unitthat notifies a node(s) adjacent to the own node among the group ofnodes of the first topology information; and an in-band communicationunit that notifies the control apparatus of the first topologyinformation, and wherein the control apparatus comprises: a topologyconfiguration unit that generates second topology information bycombining a plurality of pieces of the first topology informationnotified by the plurality of forwarding nodes; and an in-band controlunit that derives a path from the control apparatus to the plurality offorwarding nodes based on the second topology information andestablishes a control channel for controlling the plurality offorwarding nodes along the derived path, wherein the topology holdingunit updates the first topology information held in the own node basedon the first topology information received from the node(s) adjacent tothe own node among the plurality of forwarding nodes, and wherein thetopology notification unit uses, as a condition for notifying the firsttopology information to the node(s) adjacent to the own node among thegroup of nodes, at least one of: the first topology information held bythe topology holding unit has been updated; a predetermined time periodhas passed since the update; and a predetermined time period has passedsince a previous notification.
 4. The communication system according toclaim 3, wherein the control apparatus further comprises a conditioninstruction unit that specifies a condition for the plurality offorwarding nodes via the control channel.
 5. A forwarding node among aplurality of forwarding nodes controlled by a control apparatus, theforwarding node comprising: a topology holding unit that holds as firsttopology information connection relationship between an own node and atleast a part of a group of nodes including the control apparatus and theplurality of forwarding nodes; a topology notification unit thatnotifies a node(s) adjacent to the own node among the group of nodes ofthe first topology information; and an in-band communication unit thatnotifies the control apparatus of the first topology information,wherein the topology holding unit updates the first topology informationheld in the own node based on the first topology information receivedfrom the node(s) adjacent to the own node among the plurality offorwarding nodes, and wherein the topology holding unit holds, as thefirst topology information, a connection relationship between the ownnode and a node(s) within a predetermined hop number from the own nodeamong the group of nodes.
 6. The forwarding node according to claim 5,configured to receive an instruction of the predetermined hop numberfrom the control apparatus via the control channel.
 7. A forwarding nodeamong a plurality of forwarding nodes controlled by a control apparatus,the forwarding node comprising: a topology holding unit that holds asfirst topology information connection relationship between an own nodeand at least a part of a group of nodes including the control apparatusand the plurality of forwarding nodes; a topology notification unit thatnotifies a node(s) adjacent to the own node among the group of nodes ofthe first topology information; and as in-band communication unit thatnotifies the control apparatus of the first topology information,wherein the topology holding unit updates the first topology informationheld in the own node based on the first topology information receivedfrom the node(s) adjacent to the own node among the plurality offorwarding nodes, and wherein the topology notification unit uses, as acondition for notifying the first topology information to the node(s)adjacent to the own node among the group of nodes, at least one of: thefirst topology information held by the topology holding unit has beenupdated; a predetermined time period has passed since the update; and apredetermined time period has passed since a previous notification. 8.The forwarding node according to claim 7, configured to receive aninstruction of the condition from the control apparatus via the controlchannel.
 9. A control apparatus for controlling a plurality offorwarding nodes, wherein the plurality of forwarding nodes hold, asfirst topology information, a connection relationship between an ownnode and at least of a group of nodes including the plurality offorwarding nodes and the control apparatus, and notify the controlapparatus and a node(s) adjacent to the own node among the group ofnodes of the first topology information, and the control apparatuscomprises: a topology configuration unit that generates second topologyinformation by combining a plurality of pieces of the first topologyinformation notified by the plurality of forwarding nodes; and anin-band control unit that derives a path from the control apparatus tothe plurality of forwarding nodes based on the second topologyinformation and establishes a control channel for controlling theplurality of forwarding nodes along the derived path, wherein theplurality of forwarding nodes hold, as the first topology informationconnection, a relationship between the own node and a node(s) within apredetermined hop number from the own node among the group of nodes. 10.The control apparatus according to claim 9, further comprising: acondition instruction unit that specifies the predetermined hop numberfor the plurality of forwarding nodes via the control channel.
 11. Acontrol apparatus for controlling a plurality of forwarding nodes,wherein the plurality of forwarding nodes hold, as first topologyinformation, a connection relationship between an own node and at leastpart of a group of nodes including the plurality of forwarding nodes andthe control apparatus, and notify the control apparatus and a node(s)adjacent to the own node among the group of nodes of the first topologyinformation, and the control apparatus comprises: a topologyconfiguration unit that generates second topology information bycombining a plurality of pieces of the first topology informationnotified by the plurality of forwarding nodes; and an in-band controlunit that derives a path from the control apparatus to the plurality offorwarding nodes based on the second topology information andestablishes a control channel for controlling the plurality offorwarding nodes along, the derived part, wherein the plurality offorwarding nodes update the first topology information held in the ownnode based on the first topology information received from the node(s)adjacent to the own node among the plurality of forwarding nodes, andwherein the plurality of forwarding nodes use, as a condition fornotifying the first topology information to the node(s) adjacent to theown node among the group of nodes, at least one of: the first topologyinformation held by the topology holding unit has been updated; apredetermined time period has passed since the update; and apredetermined time period has passed since a previous notification. 12.The control apparatus according to claim 11, further comprising: acondition instruction unit that specifies a condition for the pluralityof forwarding nodes via the control channel.
 13. A control method forcontrolling a plurality of forwarding nodes using a control apparatus,the control method comprising: by the plurality of forwarding nodes,holding as first topology information connection relationship between anown node and at least a part of a group of nodes including the pluralityof forwarding nodes and the control apparatus; notifying a node(s)adjacent to the own node among the group of nodes of the first topologyinformation; notifying the control apparatus of the first topologyinformation; by the control apparatus, generating second topologyinformation by combining a plurality of pieces of the first topologyinformation notified by the plurality of forwarding nodes; deriving apath from the control apparatus to the plurality of forwarding nodesbased on the second topology information; establishing a control channelfor controlling the plurality of forwarding nodes along the derivedpath; and by the plurality of forwarding nodes, updating the firsttopology information held in the own node based on the first topologyinformation received from the node(s) adjacent to the own node among theplurality of forwarding nodes, wherein the plurality of forwarding nodeshold as the first topology information connection relationship betweenthe own node and a node(s) within a predetermined hop number from theown node among the group of nodes.
 14. A control method for controllinga plurality of forwarding nodes using a control apparatus, the controlmethod comprising: by the plurality of forwarding nodes, holding asfirst topology information connection relationship between an own nodeand at least a part of a group of nodes including the plurality offorwarding nodes and the control apparatus; notifying a node(s) adjacentto the own node among the group of nodes of the first topologyinformation; notifying the control apparatus of the first topologyinformation; by the control apparatus, generating second topologyinformation by combining a plurality of pieces of the first topologyinformation notified by the plurality of forwarding nodes; deriving apath from the control apparatus to the plurality of forwarding nodesbased on the second topology information; establishing a control channelfor controlling the plurality of forwarding nodes along the derivedpath; and by the plurality of forwarding nodes, updating the firsttopology information held in the own node based on the first topologyinformation received from the node(s) adjacent to the own node among theplurality of forwarding nodes, wherein the plurality of forwarding nodesuse, as a condition for notifying the first topology information to thenode(s) adjacent to the own node among the group of nodes, at least oneof: the first topology information held by the own node has beenupdated; a predetermined time period has passed since the update; and apredetermined time period has passed since a previous notification.